1. Field of the Invention
The present invention relates to a method for producing a Group III nitride semiconductor light-emitting device, and more particularly to a method for forming a p contact layer.
2. Background Art
In recent years, there has been an expanding demand for general illumination applications of Group III nitride semiconductor light-emitting devices, and high-power light-emitting devices are being developed. Every year, the driving current is increasing with the development of high-power light-emitting devices. Therefore, the power consumption is remarkably increased due to resistance, causing reduction in light emission performance. To avoid this, the resistance of light-emitting devices must be reduced.
Most of Group III nitride semiconductor light-emitting devices currently produced are of a lateral conductive face-up type or flip-chip type. In the face-up type device, an ITO transparent electrode is formed on almost the entire top surface of the p contact layer. In the flip-chip type device, a reflecting electrode of high-reflectivity metal such as Ag or its alloy is formed on almost the entire top surface of the p contact layer. To reduce the resistance of light-emitting devices, the contact resistance between the p contact layer and the electrode may be reduced. Improved light emission performance and uniform light emission distribution can be expected by reducing the contact resistance.
In a known method to reduce the contact resistance between the p contact layer and the electrode, p-InGaN is used as a p contact layer instead of conventionally known p-GaN. InGaN has a smaller work function than GaN, and has a work function closer to that of the electrode material. Thereby, the contact resistance can be reduced. Japanese Patent Application Laid-Open (kokai) No. 2010-62254 discloses a p-type contact layer comprising a p-type first contact layer formed of p-InGaN and a p-type second contact layer formed of p-GaN in this order from and on a p electrode. Such a structure of the p contact layer can reduce the contact resistance between the p contact layer and the electrode. A method is also well known to restrain evaporation of In and improve crystallinity of InGaN by employing nitrogen as a carrier gas when forming InGaN by MOCVD.
To reduce the resistance of the device, p-type activation is required for a Group III nitride semiconductor doped with Mg. This is usually done by thermal treatment in an atmosphere containing no hydrogen.
However, this method may degrade crystallinity. A method disclosed by Japanese Patent Application Laid-Open (kokai) No. 2005-159341 is also suggested. Japanese Patent Application Laid-Open (kokai) No. 2005-159341 discloses a method for p-type activation of a Group III nitride semiconductor without degradation of crystallinity, the method comprising: forming a Group III nitride semiconductor doped with a p-type dopant in a hydrogen or ammonia atmosphere at a temperature of 1000° C. or higher, lowering the temperature by 50° C. or more to maintain at 900° C. or higher, and thereafter stopping the supply of hydrogen or ammonia to replace with an inactive gas such as nitrogen.
Even with the method disclosed by Japanese Patent Application Laid-Open (kokai) No. 2010-62254 or 2005-159341, however, difficulty is encountered in reducing the contact resistance with the electrode while achieving p-type conduction of the p contact layer formed of InGaN. The resistance of the device cannot be sufficiently reduced. Therefore, both the resistance of the p contact layer and the contact resistance with the electrode must be reduced.